It is argued that Terra Australis Incognita, the ‘imaginary’ southern continent which appears on virtually all early maps of the world, was a real continent that matches Plato’s mythical Atlantis in many respects. Maps showing three different forms of Terra Australis suggest a continent of which the central plain was gradually being flooded before it ultimately disappeared under the water. The submarine topography of New Zealand closely matches one of these maps and ancient legends of South America suggest that disappearance of the continent was caused by the impact of a comet. It is argued that Antarctica must have been ice free up to 12,000 years ago, when the impact of a comet caused a tsunami which nearly caused the extinction of mankind, the tsunami having been recorded in the Bible as the Great Flood.